Semiconductor quantum dots, which are in short also referred to as dots in this specification, comprise a semiconductor structure with extensions in all three dimensions of space, which restrict the mobility of one or more charge carriers in the semiconductor material of the quantum dot to an extent, that the one or more charge carriers assume quantized states with respect to all three dimensions of space. As is well known, charge carriers in semiconductors, i.e., electrons and holes, can be described by complementary physical models as having particle properties or wave properties. From a quantum-mechanical point of view, the wave functions of the one or more charge carriers within the quantum dot are restricted in their extension in space to a value, which is smaller than the De-Broglie wavelength of the charge carriers. The De-Broglie wavelength is defined as the ratio of Planck's constant h and the momentum p of the particle.
The restriction of the mobility (in the particle aspect) or wave function (in the wave aspect) of the charge carriers is achieved by embedding the semiconductor quantum dot into a barrier. The barrier is a material with an energy gap between the valance band and the conduction band, which energy gap is larger than that of the semiconductor material of the quantum dot. The energy gap is in the art called the band gap. In other words, the barrier can be a semiconductor with a larger band gap than the semiconductor of the quantum dot, or it can be an insulator.
Semiconductor quantum dots have advantageous electronic and optical properties. For instance, optoelectronic applications like light-emitting diodes or laser diodes can profit from an increased efficiency of radiative recombination of electrons and holes in a quantum dot. This can be achieved by tailoring the size, the shape and the material of the quantum dot so as to increase the overlap of the wave functions of electrons and holes within the quantum dot. An increased wave-function overlap raises the probability of radiative recombination. It also increases the probability of light absorption by the quantum dot in comparison with bulk material, which is an advantage for detector applications. Another application of quantum dots is their use as single-photon emitters, for instance in cryptographical applications.
The quantum dots can be designed by proper choice of material composition, geometrical shape and size to emit or absorb light in a desired spectral range.
However, quantum dots are also interesting for purely electronic applications, including memories or transistors. By proper design, quantum dots can be configured to store a charge for a very long time. That is, an electron-hole pair representing an information bit can be generated in certain types of quantum dots, wherein the wave-functions of the electron and the hole have a very poor overlap, thus deliberately keeping the probability of recombination low for the purpose of storing the information bit.
Semiconductor quantum-dot devices have attracted considerable interest in research and development since the early 1990s when self-organized quantum-dot formation on a substrate in the Volmer-Weber and, in particular, Stranski-Krastanov growth modes was employed as a reliable hetero-epitaxial technique for fabricating quantum dots of rather homogeneous shape and size. However, a self-organized growth on nanostructures has the disadvantage that it is difficult to control the growth of a semiconductor quantum dot at an exact desired lateral position on a substrate.
In many applications, however, it is in fact desired to control the exact lateral position of the quantum dots on a substrate. One method for fabricating quantum dots with position control is disclosed in US 2005/0233487 A1. A disadvantage of this method is that it requires a multiple patterning steps of a complex layer structure with lithographic techniques. In addition, the patterning must be applied in two lateral directions.
It would be desirable to provide a simpler process for position-controlled fabrication of a semiconductor quantum dot. It would also be desirable to provide a semiconductor quantum-dot device structure, in which the semiconductor quantum dots can be fabricated in a position-controlled process at low complexity and cost.